Optical dielectric waveguides are of great interest at present in the field of communications, as they are potentially capable of conveying large amounts of information per unit time over relatively long stage lengths.
Those types of optical dielectric waveguides so far proposed, that are potentially suitable for long distance optical communication systems, consists essentially of a fibre with a low loss core surrounded or "cladded" by a sheath of transparent material with a refractive index of 1% or more less than that of the core. In order to be able to make such fibres it is necessary that the material of the core and sheath have compatible thermal and mechanical properties. In single mode cladded fibres the core has a diameter of a few microns and this allows only the HE.sub.11 mode to propagate. In multimode cladded fibres the core diameter is about 50 microns or larger. The outer diameter of the sheath is not critical provided it is sufficiently larger than that of the core.
Over distances of about a kilometer, multimode fibres can have bandwidths of up to about 1 GHz. However certain materials have such low losses that stage lengths of some 20 kilometers are possible. Over such large distances the bandwidths of multimode fibres become only about 20 MHz or less. This bandwidth can be increased under some circumstances by allowing a certain amount of intermode conversion to occur but this usually leads to an increase in the loss of the fibre. Single mode fibres on the other hand can have bandwidths of up to about 50GHz over a distance of a kilometer and of about 11 GHz over a distance of 20 Kilometers if the carrier frequency spread is sufficiently small. If the carrier frequency spread is about 400 GHz however these possible bandwidths are reduced to about 6GHz and 300 MHz respectively. Even so a single mode fibre has a capacity greatly in excess of that of multimode fibres over distances of interest for long range high capacity communication systems.
A major difficulty in making cladded fibres is to find two suitable low loss materials with compatible thermal and mechanical properties. At present pure silica with a loss of about 2dB per kilometer has the lowest loss yet known. The only low loss material known at the moment that is suitable for cladding pure silica is a high boron content binary borosilicate gloss. Multimode fibres of this type have been made with losses of about 7dB per kilometer. An alternative is to clad a core, made of silica doped with suitable impurities to raise its refractive index slightly, with pure silica. Multimode fibres have been made in this way with losses as low as 2dB per kilometer. Doping a material in this way can however substantially increase its material dispersion. Thus single mode cladded fibres with cores doped in this way could have rather lower bandwidths than might otherwise be expected.
A variant of the cladded fibre in which there is no material compatibility problem is the liquid filled multimode fibre. Such fibres are made by filling hollow fibres with a low loss fluid having a sufficiently high refractive index. Liquid filled single mode fibres are not a practical proposition because of the inordinate time it would take to fill them with liquid. A second variant of the cladded fibre that also has no material compatibility problem consists of a hollow fibre with an inner fibre running along its centre axis and supported by a thin flat plate, all the components being made of the same low loss material. In this case the inner fibre forms the core, and air, the thin plate and to a lesser extent the outer fibre act as the cladding. This type of fibre can be used in either multimode or single mode form according to the size of the inner fibre. Multimode fibres of this type have been made out of pure silica with a loss of about 5dB per kilometer.
Single mode cladded fibres have so far nearly always been found to have substantially higher losses than the corresponding multimode cladded fibres. As at least a fifth of the energy of a single mode fibre is carried in the cladding this is easy to understand when the cladding has a much higher loss than the core. When this is not the case the reason is probably due to the fact that while intermode conversion does not necessarily increase the loss in multimode cladded fibres, in single mode cladded fibres it always does. Thus inhomogeneities in the core and cladding as well as irregularities in the core cladding interface (where the intensity is high in single mode cladded fibres) are the probable cause of the increased loss of single mode cladded fibres.